NIR-FT Raman and FT-IR spectral studies and ab initio calculations of the anti-cancer drug combretastatin-A4

Combretastatin-A4, a potential anti-cancer drug in advanced preclinical development, is extracted from the medicinal plant Combretum caffrum. NIR-FT Raman and FT-IR spectral studies were carried out and ab initio calculations were performed at the HF/6-31G(d) level to derive equilibrium geometry and vibrational wavenumbers and intensities. The vibrational analysis shows that the molecule exhibits similar geometric behavior to cis-stilbene, and has undergone steric repulsion resulting in phenyl ring twisting with respect to the ethylenic plane. The spectral behavior of the methoxy groups is influenced by the oxygen lone pair-aryl p(z) orbital conjugation and back-donation. Chelation of the OH group with the oxygen lone pair of the methoxy group produces separate bands for free OH and associated OH vibrations. Copyright (C) 2004 John Wiley Sons, Ltd.